1. Field of the Invention
The present invention relates to a method for fabricating a magnetic recording medium, and more particularly, relates to a method for fabricating a magnetic recording medium to be installed in a fixed magnetic disc unit or the like employable as an external memory unit for an information processing apparatus such as a computer or the like.
2. Description of the Prior Art
In general, a magnetic recording medium is fabricated in such a manner that a magnetic thin film serving as a magnetic recording layer is first formed on a surface of a non-magnetic base plate and a hard protective film is then deposited on the magnetic thin film. A non-magnetic base plate which has been obtained by finishing a disc-shaped plate composed of an aluminum alloy, a reinforced glass, a plastic material, an amorphous carbon or the like to assume a predetermined degree of parallel, flatness and surface roughness may be employed. Alternatively, a non-magnetic base plate which has been obtained by forming a surface layer on the surface thereof by performing alumilite processing or Ni-P alloy electroless plating processing as desired and then finishing the surface layer to assume a required surface roughness is used. In addition, an underlayer for enhancing the magnetic properties of the magnetic thin film, e.g., a chromium layer is additionally formed on the non-magnetic base plate as desired. A Co-Ni-P alloy layer serving as a magnetic recording layer is formed on the non-magnetic base plate by employing a electroless plating process. Alternatively, a magnetic metallic film such as a cobalt alloy film or the like or a magnetic oxide film such as a .gamma.-Fe.sub.2 O.sub.3 film or the like is formed on the non-magnetic base plate by employing a sputtering process and so on. A thin film composed of graphite, SiO.sub.2 or the like is formed on the magnetic thin film as a hard protective film of the aforementioned type by employing a sputtering process, a spin coating process or the like. Further, a liquid lubricant for improving lubricating and wear resisting properties of the magnetic recording medium is coated on the hard protective film as desired.
With respect to the fixed magnetic disc unit, an information reading/writing operation is performed for the aforementioned magnetic recording medium with the aid of a magnetic head. Usually, a contact start stop process (hereinafter referred to as a CSS process) is employed for performing the information reading/writing operation. To practice the CSS process, the magnetic head is caused to run while it is floated by a small height (about 0.1 .mu.m to 0.3 .mu.m) by a function induced by an air stream appearing between the magnetic head and the magnetic recording medium when the magnetic disc is driven for performing the information reading/writing operation. On the contrary, when the disc is stationary, the magnetic head is brought in contact with the surface of the magnetic recording medium. When driving of the disc is started, the magnetic head slidably moves along the surface of the magnetic recording medium which has started its rotation. Similarly, when driving of the disc is stopped, the magnetic head slidably moves along the surface of the magnetic recording medium which has stopped its rotation. For this reason, to assure that an information reading/writing operation is smoothly performed, it is required that no abnormal projection is present on the surface of the magnetic recording medium so as not to obstruct stable running of the magnetic head in a floated state, and moreover, the surface of the magnetic recording medium is adequately finely coarsened so as to maintain a low coefficient of friction without any occurrence of a malfunction such as hot seizure of the magnetic head during slidable movement of the same.
To meet the foregoing requirement, surface irregularities are usually formed on the surface of the non-magnetic base plate by employing a texture working process or the like after the surface of the non-magnetic base plate is worked by performing a grinding operation or the like so as to assume a very fine surface roughness approximate to a mirror surface. To this end, a magnetic recording medium is produced by using a non-magnetic base plate which has been worked to assume such a surface roughness as represented by an index .DELTA.Cv (10%-1%) of which value is about 0.01 .mu.m to 0.02 .mu.m or more wherein the index .DELTA.Cv (10%-1%) is derived from subtraction of a cutting depth corresponding to one percent bearing length ratio from a cutting length corresponding to ten percent bearing length ratio on the curve of a profile bearing length ratio as specified in accordance with ISO (International Standard) 4287/1--1984. When an underlayer, a magnetic layer and a protective layer are successively formed on the surface of the base plate having fine projection as mentioned above by employing a sputtering process under suitable sputtering conditions, fine projections corresponding to fine projections on the surface of the non-magnetic base plate or increased surface irregularities are formed on the surface of the magnetic recording medium. As a result, good sliding of the magnetic head and stable running of the magnetic head in a floated state can practically be realized.
A number of fine projections have been hitherto formed on the surface of the magnetic recording medium in the above-described manner because of a necessity for reducing the coefficient of friction of the surface of the magnetic recording medium.
In recent years, as the amount of information to be processed increases, a fixed magnetic disc unit has been positively designed and constructed in such a manner as to have a larger capacity. However, when a magnetic head is caused to run in a very low floated state (with a floatation of, e.g., 0.1 .mu.m or less) so as to allow the recording of information on a magnetic recording medium with a higher recording density or reading information recorded with a higher recording density, in view of the aforementioned current tendency, there arises a problem in that it is difficult to run the magnetic head in a stably floated state against a magnetic recording medium which is produced by employing a conventional method wherein a number of fine projections are previously formed on the surface of a base plate. To practically realize stable running of the magnetic head in a very low floated state, it is necessary that the height of each projection be reduced much more than that obtainable with the conventional method. However, a difficulty is that the fine projections mentioned above are formed while their height is made uniform in order to prevent formation of projections each having an abnormal height which may allow head crush to take place. In addition, since the base plate is made of a hard material, it is likewise difficult that head parts of the projections each having an abnormal height are cut off at a subsequent step so as to make their height.
Additionally, a proposal has been made with respect to a method of uniformly forming a number of fine projections each having a predetermined height on the surface of a magnetic recording medium wherein the method is practiced by way of steps of previously forming the fine projections each having a low height on a surface of a base plate, locally abnormally growing a film while a hydrocarbon base organic material distributed on the surface of the base plate is used as a core, enlarging the height of the projections formed on the surface of a magnetic recording medium, and thereafter, making uniform the height of each projection by employing, e.g., a tape burnishing process. With respect to the method as mentioned above, since head parts of the projections are made of a carbon that is a comparatively soft material, this makes it possible to employ a tape burnishing process. However, in view of the current tendency of responding to the requirement for the higher recording density, a material having a high coercive force and with which a low noise is realized is required as a material for the magnetic layer of the magnetic recording medium. As a result, the Co-Ni-Cr based alloy which has been mainly used at present is being shifted to a Co-Cr-Ta based alloy or a Co-Cr-Pt based alloy. An especially significant feature of the magnetic recording medium including a magnetic layer made of the foregoing material is that it exhibits low noise, and this is attributable to the fact that crystal grains are increasingly isolated due to segregation of chromium in the magnetic layer. As a result, there appears a drawback that it is difficult to improve the squareness of the magnetic hysteresis loop. However, to practically improve the squareness, it is necessary when the magnetic film is sputter grown on a base plate that the temperature of the base plate is elevated or a bias voltage is applied to the base plate. Conclusively, it is difficult to practically employ a method of forming a number of required fine projections on the surface of a magnetic recording medium by maintaining the temperature of a base plate at a low level during the film forming process as mentioned above.